Xylene isomers, para-xylene, meta-xylene and ortho-xylene, are important intermediates which find wide and varied application in chemical syntheses. Para-xylene upon oxidation yields terephthalic acid, which is used in the manufacture of synthetic textile fibers and resins. Meta-xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Ortho-xylene is feedstock for phthalic anhydride production.
Xylene isomers from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates, and further comprise ethylbenzene, which is difficult to separate or to convert. Para-xylene in particular is a major chemical intermediate with rapidly growing demand, but amounts to only 20 to 25% of a typical C8 aromatics stream. Among the aromatic hydrocarbons, the overall importance of the xylenes rivals that of benzene as a feedstock for industrial chemicals. Neither the xylenes nor benzene are produced from petroleum by the reforming of naphtha in sufficient volume to meet demand, and conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Often toluene (C7) is dealkylated to produce benzene (C6) or selectively disproportionated to yield benzene and C8 aromatics from which the individual xylene isomers are recovered.
A current objective of many aromatics complexes is to increase the yield of xylenes and to de-emphasize benzene production. Demand is growing faster for xylene derivatives than for benzene derivatives. Refinery modifications are being effected to reduce the benzene content of gasoline in industrialized countries, which will increase the supply of benzene available to meet demand. A higher yield of xylenes at the expense of benzene thus is a favorable objective, and processes to transalkylate C9 and heavier aromatics with benzene and toluene have been commercialized to obtain high xylene yields.
U.S. Pat. No. 4,857,666 discloses a transalkylation process over mordenite and incorporating a metal modifier into the catalyst.
U.S. Pat. No. 5,763,720 discloses a transalkylation process for conversion of C9+ into mixed xylenes and C10+ aromatics over a catalyst containing zeolites including amorphous silica-alumina, MCM-22, ZSM-12, and zeolite beta, where the catalyst further contains a Group VIII metal such as platinum.
U.S. Pat. No. 6,060,417 discloses a transalkylation process using a catalyst based on mordenite with a particular zeolitic particle diameter and having a feed stream limited to a specific amount of ethyl containing heavy aromatics. The catalyst contains nickel or rhenium metal.
U.S. Pat. No. 6,486,372 B1 discloses a transalkylation process using a catalyst based on dealuminated mordenite with a high silica to alumina ratio that also contains at least one metal component.
U.S. Pat. No. 6,613,709 B1 discloses a catalyst for transalkylation comprising zeolite structure type NES and metals such as rhenium, indium, or tin.
U.S. Pat. No. 6,740,788 B1 discloses an integrated process for aromatics production enabled by a stabilized transalkylation catalyst having a metal function.
Many types of supports and elements have been disclosed for use as catalysts in processes to transalkylate various types of aromatics into xylenes, but there exists a problem presented by transalkylation aromatics feed stream contaminants, whereby such contaminants reduce the useful catalyst cycle life. Applicants have found a solution with the application of a contaminant removal guard bed that extends catalyst life, resulting in a more stable aromatics transalkylation process that will be more profitable over the catalyst life cycle by requiring less frequent down time for regeneration to remove deactivating coke deposits.